The automotive industry has used various gas sensors in automotive vehicles for many years. For example, electrochemical sensors based on polarographic principles have been developed for determining the concentration of oxygen or unburned components in exhaust gases produced by an internal combustion engine or a motor vehicle. These types of oxygen sensors typically include a pump cell and a Nernst cell built, for example, from solid oxide electrolyte materials such as doped zirconia, and linked together through an external electrical circuit. The Nernst cell includes an air reference electrode (or a biased reference electrode) and a sensing electrode with a solid electrolyte therebetween. The pump cell includes a first and second electrode with a solid electrolyte therebetween and a gas chamber with an aperture. The first electrode of the pump cell and the sensing electrode of the Nernst cell are exposed to the gas chamber that receives a representative flow of test gas, such as engine exhaust gas. A controlled electrical potential is applied to the pump cell to pump oxygen into and out of the gas chamber to maintain the electromotive force of the Nernst cell as sensed at the air reference electrode thereof at a desired potential.
To provide for sensing of the oxygen concentration in the test gas, such as by sensing oxygen flux in the gas chamber, the sensor must be maintained in a current limiting range of operation by maintaining the Nernst potential applied to the sensor within a predetermined voltage range. The current limiting range of operation is characterized by a sensor output current that is insensitive to variations in the potential applied to the pump cell. In such a range of operation, the aperture limits gas flux into or out of the gas chamber and sensor output current indicates the maximum flow that can be supported by the concentration in the test gas. If the potential is above the predetermined Nernst voltage range, additional oxygen may be stripped from gas species such as water (H2O) and carbon dioxide (CO2), skewing the relationship between the gas concentration and sensor output current. If the potential is below the predetermined Nernst voltage range, an excess of oxygen is available and sensor output current does not indicate oxygen concentration but rather is a nonlinear function of the gas concentration.
Current sensors such as the oxygen sensors described above are inadequate for determining hydrogen concentration in an air environment. For example, when the sensor is operating in an air environment, it needs to operate at a very high temperature, typically above 700° C. to be able to pump all the oxygen.
There are also solid electrolytes that can conduct protons instead of oxygen ions. They can substitute the zirconia electrolyte of the above mentioned device in order to perform hydrogen or hydrocarbon (linear) sensing in air. However, these electrolytes are not chemically stable and will dis-integrate during sensing operation. Additionally, the chemically stable proton conducting electrolytes are not conductive enough to pump protons to the limiting value without getting into the electrolysis range. Thus, it is desirable to have a hydrogen sensing device that is stable, is operable at temperatures below about 600° C., and preferably that is sensitive to hydrogen or hydrocarbon concentrations, or to other gases such as CO, in air environment.